41. petrography of barremian synrift … · tion of sedimentary rocks. ... petrography of barremian...

41. PETROGRAPHY OF BARREMIAN SYNRIFT SEDIMENTS, SITE 549, GOBAN SPUR1

Jim Mazzullo, Elsa K. Mazzullo, and Kim Henke, Department of Geology, Texas A & M University2

ABSTRACT

During Leg 80 of the Deep Sea Drilling Project, a 290-m section of calcareous and noncalcareous Barremian synriftsediments was drilled in Hole 549. A petrographic study of samples from this section was undertaken in an attempt todetermine the general conditions under which these sediments accumulated.

The deposition of the Barremian synrift sediments apparently occurred in relatively shallow, semirestricted to open-marine environments during a marine transgression. The basal unit (Unit 10) consists of fine-grained terrigenous andcalcareous debris deposited in very shallow nearshore waters, possibly in a lagoon, bay, or protected shelf area. The unitabove (Unit 9), characterized by milliporoid boundstone and skeletal grains, represents sedimentation at a high-energy,well-oxygenated shelf margin. The uppermost unit, consisting of fossiliferous wackestone grading upward to nonfossil-iferous micritic mudstone, represents relatively deep-water sedimentation in foreslope environments seaward of the shelfmargin.

INTRODUCTION

During Leg 80 of the Deep Sea Drilling Project, a thin(290.65-m) section of calcareous and noncalcareous Bar-remian synrift sediments overlying a basement high wasdrilled in Hole 549. This section was subdivided intofour lithologic units and various subunits on the basisof the shipboard description of lithology and sedimen-tary structures, and from these data a general model forthe deposition of the sequence was formulated.

This paper reports an additional, more detailed studyof samples from this same section. It serves to supple-ment our shipboard descriptions of the units and subunitswith thin-section, X-ray, and scanning electron micros-copy (SEM) data and to refine our model for the originof the section. It also serves to evaluate the accuracy ofsmear-slide data generated on board ship in the descrip-tion of sedimentary rocks.

This study is based on the analysis of 21 samples tak-en by a member of the scientific staff of Leg 80 (J. M.)during the cruise. These samples were carefully chosen torepresent all the major lithologic types and units or sub-units described in the synrift section by the scientific staffof that leg.

PETROGRAPHY OF BARREMIAN SYNRIFTSEDIMENTS

The Barremian synrift sediments in Hole 549 are di-vided into units and subunits on the basis of onboard de-scriptions of lithology and sedimentary structures. Forease of comparison with the site chapter (this volume),we consider each of these unit or subunits, and our sam-ples from each, separately. We note at the start that ouranalyses of the sediments support the breakdown of thesynrift section made by the scientific staff.

Graciansky, P. C. de, Poag, C. W., et al., Init. Repts. DSDP, 80: Washington (U.S.Govt. Printing Office).

2 Address: (J. Mazzullo) Department of Geology, Texas A & M University, College Sta-tion, TX 77843; (E. K. Mazzullo, present address) Ocean Drilling Program, Texas A & MUniversity; (K. Henke, present address) Exxon, Inc., Kingsville, TX.

In the laboratory, each sample was split into three piec-es: one piece was impregnated with resin and ground in-to a thin section from which textural and compositionaldata were gathered. A second piece was used to preparean oriented mount for X-ray determination of the claysuite, and a third piece was mounted on an aluminumplug, coated with a gold-palladium alloy, and examinedwith a JSM-25S SEM to determine clay morphology.

Unit 8This unit (673.85-755 m BSF) is a thick, upward-fin-

ing section of gray interbedded calcareous mudstones andsandy calcareous mudstones overlain by a thin unit ofvariegated calcareous mudstone.

Two samples from the sandy calcareous mudstone li-thology (60-6, 76-78 cm and 61-3, 30-32 cm) were ex-amined (Fig. 1). The sand content is high in these sam-ples: 45 and 41%, respectively. It consists of bioclasticdebris (22 and 19.5%, respectively), which specificallyinclude recrystallized bivalve fragments (8 and 9.5%),echinoderms (5 and 7%), foraminifers (1 and 6%), brachi-opods (<2%), molluscs (0 and 1.5%), and ostracods(<1%); as well as quartz (15 and 7%); micritic pellets(7 and 3%); and traces of feldspar. The matrix is com-posed of micrite and sparite (together totaling 40 and43%, respectively, of the two samples), clays (mainly il-lite and chlorite, together totaling 10 and 1%, respec-tively, of the two samples), mica (0.5%), pyrite (1 and2%), and plant material (2 and 4%). Both samples alsoshow extensive hematite and limonite staining. A thirdsample from this lithology (59-1, 74-76 cm) contains con-siderably more clay (41%) and bioclastic debris (50%),and lesser amounts of quartz (7%), pyrite (1%), feldspar,and opaque minerals.

In contrast, three samples from the gray calcareousmudstone lithology (54-3, 128-131 cm, 55-4, 121-124cm, and 56-3, 95-98 cm) (Fig. 2) are composed largelyof micrite (70 and 87%), with lesser amounts of micriticpellets (0-15%); skeletal fragments (1 to 7%), which spe-cifically include echinoderms (0.5-1.5%), brachiopods

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J. MAZZULLO, E. K. MAZZULLO, K. HENKE

Figure 1. Sandy calcareous mudstone of Unit 8, Sample 60-1, 76-78 cm. Note the abundance of terrigenousand bioclastic sand debris. Crossed nicols, ×25.

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(0.5-1.0%), foraminifers (0-3%), unidentifiable bio-clasts (0.5-4%), and traces of bryozoans, pelecypods, al-gae, sponge spicules, and gastropods; as well as quartz(1.5-12%); clay (1-8%), which is largely illite, mont-morillonite, and mixed-layer illite-montmorillonite;plant material (1%); and frambroidal pyrite (1%).Patchy hematite stains were seen throughout the matrixand on pellets. Total mud content for these samples var-ies between 76 and 94%.

Lastly, two samples from the variegated calcareousmudstones at the top of the unit (Core 53) consist large-ly of hematite-stained micrite and clay (mostly illite). InSample 53-1, 40-43 cm, the amounts of these two com-ponents were calculated as 16 and 51%, respectively; inSample 53-2, 64-67 cm, they were optically indistinguish-able because of unusually heavy hematite staining, buttogether totaled 70%. The remainder of this lithologyconsists of hematite-stained skeletal debris (0-3%), mi-

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PETROGRAPHY OF BARREMIAN SYNRIFT SEDIMENTS

critic pellets (0-15%), and quartz (4-15%). The totalamount of mud in both samples varies between 78 and82%. In Sample 53-2, 64-67 cm, sand-sized dolomiterhombohedrons (Fig. 3) recrystallized from the carbon-ate mud were abundant (26%).

Unit 9Recovery from this unit (755-801.5 m BSF) was ex-

tremely poor, owing to the brittle nature of the sedi-ment, and hence no samples were available for post-cruise study. The few samples available for study on

board ship are described in the Site 549 chapter (thisvolume). Basically, this unit consists of carbonate grain-stones and lesser amounts of algal mudstones.

Unit 10, Subunit aSubunit 10a (801.5-843 m BSF) was described on board

ship as gray calcareous sandy mudstones and wacke-stone interbedded with noncalcareous mudstones andsandy mudstones.

A thin section of the carbonate wackestone lithology(74-3, 75-78 cm) (Fig. 4), which predominates in Core

Figure 3. Authigenic dolomite rhombohedrons in Sample 53-2, 64-67 cm. Note also the darkhematite stain in the mud surrounding the dolomite. Plane light, × 120.

Figure 4. Carbonate wackestone in Subunit 10a, Sample 74-3, 75-78 cm. Note abundance of shell debrissupported in a sparry matrix. Crossed nicols, × 120.

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72 and is found periodically throughout Cores 74 to 77,showed it to consist of coarse sand- to gravel-sized shelldebris (16%), small amounts of micritized pellets (3%),quartz (5%), pyrite (0.5%), and plant material (6.5%),all supported by a sparry calcite cement (62%), some ofwhich presumably recrystallized from skeletal debrisand mud. The identifiable shell debris consists of mol-lusc shells (10%) and lesser amounts foraminifers (2%),echinoderms (2%), cephalopods (1%), gastropods (0.5%),and brachiopods (0.5%). The muddier carbonate lithol-ogy (Sample 549-72,CC) is composed largely of micrite(90%) and minor amounts of quartz (8.4%), bioclasts(< 1%), and framboidal pyrite (< 1%).

The noncalcareous sandy mudstone (76-1, 61-64 cm)(Fig. 5) has a mud matrix composed of clay (69%) andmicrite (3%); SEM and X-ray analyses indicate that theclay fraction consists largely of kaolinite and illite andminor amounts of chlorite. The sand fraction consists ofquartz (18%), mica (3%), feldspar (1%), plant material(<1%), unknown bioclasts (1%), micritic pellets (3%),and pyrite (<1%). The rock as a whole is irregularlystained throughout with hematite. Lastly, the noncalcar-eous mudstone lithology (75-4, 25-28 cm) (Fig. 6) iscomposed mainly of clay (96%, mainly kaolinite, illite,and mixed-layer clays), and small amounts of sand-sizedquartz (2%), muscovite (1%), and plant material (1%).Fossil debris is absent in this lithology.

Unit 10, Subunit bSubunit 10b (843-870 m BSF) was described as con-

sisting of massive, gray, calcareous sandstones and non-calcareous mudstones and sandy mudstones.

A thin section from the calcareous sandstone litholo-gy (78-1, 92-94 cm) (Fig. 7) shows this lithology to befairly silt rich (45%): thus, a more exact name for this li-

thology would be calcareous silty sandstone. The sandand silt fraction consists largely of quartz (39%), hema-tite- and limonite-stained micritic pellets (7%), echino-derms (2%), and other unidentifiable skeletal debris (2%).Clays, particularly kaolinite and illite, make up 6% ofthe sample. The sediment is cemented with sparite (43%)and lesser amounts of hematite (0.5%) and chalcedony(0.5%).

A sample of the noncalcareous mudstone lithology(79-1, 53-55 cm) was composed almost entirely of ka-olinite, illite, and chlorite (94%), with minor amountsof quartz sand (5%) and carbonate skeletal clasts (1%).

Unit 10, Subunit cSubunit 10c (870-884 m BSF) was described on board

as consisting of variegated noncalcareous mudstones orsandy mudstones, and more rarely calcareous mudstonesor sandy mudstones. Analysis of a sample (81-2, 31-32cm) of the noncalcareous lithology indicates that it con-sists largely of kaolinite and illite (83%), with lesseramounts of sand-sized quartz (12%), pyrite (5%), andtraces of plant debris and calcite spar.

Unit 10, Subunit dSubunit lOd (884-964.5 m BSF) was described as thin

interbeds of claystone, siltstone, mudstone, sandy mud-stone, and sandstone, with an occasional thin layer ofshelly limestone.

Two samples from the mudstone lithology (88-3, 64-67 cm and 89-1, 125-128 cm) (Fig. 8) contain largeamounts of a kaolinite- and illite-dominated matrix (90and 92%, respectively), and small amounts (9 and 7%)of sand- and silt-sized sediment, mostly quartz (5 and3%), mica (1 and 2%), bioclasts (<1%), and pellets(<2%). Micrite was found in trace (0.5%) amounts in

Figure 5. Noncalcareous sandy mudstone in Subunit 10a, Sample 76-1, 61-64 cm. Note abundance ofsand-sized terrigenous debris (largely quartz). Crossed nicols, ×25.

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Figure 6. Noncalcareous mudstone lithology in Subunit 10a, Sample 75-4, 25-28 cm. Noteabundance of clay and dark hematite stain. Crossed nicols, × 120.

Figure 7. Calcareous silty sandstone of Subunit 10b, Sample 78-1, 92-94 cm. Note abundantsilt-sized quartz and sparry cement. Crossed nicols, × 120.

Sample 89-1, 125-128 cm, and chlorite was found in mi-nor amounts in Sample 88-3, 64-67 cm.

Three samples from the sandy mudstone lithology (86-2, 43-46 cm, 90-3, 3-6 cm, and 93-1, 41-43 cm) (Fig. 9)contained 21 to 25% sand- and silt-sized material, mostlyquartz (13-15%), feldspar (0-4%), micritized pellets (0-4%), unidentifiable shell fragments (0-1%), plant mate-rial (2-4.5%), and pyrite (0-4%). The matrix consistslargely of kaolinite and illite, with only trace amountsof micrite. Sample 86-2, 43-46 cm was originally de-

scribed as a "mudstone," and 93-1, 41-43 cm was de-scribed as a "sandstone." However, both are clearly"sandy mudstones."

A sample from the "shelly limestone" lithology (88-4, 0-3 cm) (Fig. 10) is more properly described as a mar-ly limestone or muddy wackestone. It contains 25.5%fossil fragments (mostly bivalve fragments) supported inan iron-stained, micritic and clay matrix that makes up70% of the rock. Quartz sand and silt make up an addi-tional 2% of this lithology. Another sample of this same

931


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Figure 8. Mudstone in Subunit lOd, Sample 88-3, 64-67 cm. Note abundance of clay and small amount ofquartz. Crossed nicols, ×25.

Figure 9. Sandy mudstone from Subunit lOd, Sample 90-3, 3-6 cm. Note the abundance ofsand- and silt-sized quartz and the clayey matrix. Crossed nicols, × 120.

lithology (85-2, 45-48 cm) described on board as a silt-stone is also a marly wackestone. This sample containslarge shell fragments (15%), pellets (3%), quartz (23%),and plant material (4.5%), all supported in a matrix ofmicrite (35%) and clay (10%, largely kaolinite and il-lite). Framboidal pyrite is also very abundant in thissample (7.5%) (Fig. 11).

During sampling of this section on board ship forthis study, samples of lithologies described as siltstone,

claystone, and sandstone were deliberately chosen. Thin-section examination, however, showed such samples tobe misnamed; most often these lithologies are more prop-erly mudstones or sandy mudstones.

Similarly, one lithology not noted in the core descrip-tions is a calcareous sandy mudstone, represented by Sam-ple 88-3, 0-3 cm (Fig. 12). This sample consists of a ma-trix of kaolinite and illite (56%) and micrite (8%) sup-porting sand-sized quartz (16%), fossil fragments (12%),

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Figure 10. Wackestone in Subunit lOd, Sample 88-4, 0-3 cm. Note abundant shell debris supported in aniron-stained matrix of micrite and clay. Crossed nicols, x25.

Figure 11. SEM imagery of framboidal pyrite, Sample 85-2, 45-48 cm; × 3000.

pellets (5%), plant material (2%), and pyrite (1%). Thissample had previously been described as a sandy mud-stone.

NOTES ON CLAY MINERALOGY

Mineralogic identification of clays was based on X-ray analyses, thin-section petrography, and SEM imag-ery. Each sample was analyzed by X-ray diffraction infour successive stages to determine the clay suite present:

untreated, glycolated, heated to 400°C, and lastly, heat-ed to 500°C (Carroll, 1978). No attempts were made toquantify the X-ray diffraction data in order to determinethe relative proportion of all the clay minerals present.Rather, after X-ray diffraction, each sample was exam-ined either in thin section or with the SEM, and only theclays present in the sample in major amounts were re-ported. Therefore, all references to changes are strictlyqualitative and should be so construed. Lastly, the mor-

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Figure 12. Calcareous sandy mudstone in Subunit lOd, Sample 88-3, 0-3 cm. Note abundance of sand-sized quartz and shell debris supported in a matrix dominated by clay. This sample was described onboard ship as a sandy mudstone. Crossed nicols, ×25.

phology and location of the clays in each sample wereexamined with the SEM to determine whether they weredetrital, authigenic, or recrystallized.

The clay suite in the synrift sediments is very diverse,with nearly every major clay group present in variableamounts. Two significant trends in clay mineral abun-dances, however, have been noted. The first is the pres-ence of large amounts of kaolinite in Cores 74 through93 and its absence in Cores above 74. (Kaolinite is con-sidered to be the dominant clay mineral in fluvial, delta-ic, and nearshore sediments [Weaver, 1958].) The secondis the presence of large amounts of montmorillonite inCores 53 through 74 and its absence in significant amountsbelow Core 74. (Montmorillonite is considered to be thedominant clay mineral in open-marine sediments.) SEMdata indicate that most of the clays in the synrift sedi-ments are detrital in origin, which is evidenced by theclays' usual occurrence as intercalated laminae, deformedby mechanical compaction (Fig. 13), or as dispersed ma-trix, sometimes grain supporting. Some of the clays,however, particularly kaolinite and to a lesser degree il-lite, have undergone recrystallization after deposition,as indicated by their well-developed morphology (Fig.14) (Wilson and Pittman, 1977).

NOTES ON SMEAR SLIDES

One of the purposes of this study was to weight theadvantages and disadvantages of the smear-slide tech-nique used on board ship to describe the texture andcomposition of sediments. With this in mind, we havecompared smear-slide descriptions of lithologic units asreported in the site chapter (this volume) with our thin-section data, and the conclusions we have reached are asfollows.

First, we note that grain size estimates from smearslides are most accurate with fine-grained, well-sorted

sediment of any composition and decrease in accuracyas grain size increases and degree of sorting lessens. Themain reason for the discrepancy with more poorly sort-ed, sandy sediments lies in the preparation of the smearslide itself. During the application of the sample to thecover glass, coarser grains are segregated to the edges ofthe slide while muds occupy the center. This introducesa bias into the visual estimate of grain size, inasmuch asthis technique assumes an even distribution of all com-ponents over the entire slide.

Second, the construction of a smear slide is a destruc-tive process. Bioclasts and other fragile components,particularly cements, are destroyed by the process; also,grain-to-matrix relationships, such as occur in wacke-stones, are not evident in smear slides. This problembecomes most acute when dealing with coarser grainedcarbonate sedimentary rocks and precludes the use ofDunham and related classification schemes for suchrocks.

Third, although composition is relatively easy to de-termine for silt- and clay-sized clasts, it is more difficultif grains are coarser, thicker, and more opaque. This isthe most common problem with the sand-sized fractionof any sample, whether clastic or carbonate.

Lastly, there is a bias in the classification scheme forsediments in favor of fine-grained, siliceous and calcare-ous, biogenic sediments and against coarse elastics andcarbonates. Chalks and oozes, for example, are very pre-cisely categorized, whereas such diverse rocks as wacke-stones, grainstones, and boundstones are all lumped to-gether into one general category "limestone". A moreinformative and detailed classification scheme for lime-stones is obviously needed.

Despite these drawbacks, the smear-slide techniqueworked relatively well with the rocks cored during Leg80. The correlation between smear-slide and thin-section

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10 µm 5µm

Figure 13. A. SEM imagery of clays in Sample 89-1, 125-128 cm, showing their occurrence as intercalatedlaminae of detrital origin subsequently deformed by postdepositional compaction. B. Close-up ofsame sample showing the well-developed morphology of the clays. Dispersive analysis of the claysidentifies them as illite.

descriptions in our postcruise study of the sediments isgenerally good. The relative proportions of componentsestimated from smear slides are often in error by at least10%, causing error in classification of the sample, butfor most of the lithologies encountered during Leg 80,smear slides resulted in good descriptions, albeit semi-quantitative and general, of samples in a short amountof time. The bulk of our stratigraphic section consistedof homogeneous, fine-grained lithologies, and we did notoften encounter coarse lithologies, but even when wedid, the comparison between smear-slide and thin-sec-tion data is favorable. We do recommend that for coars-er sediments, the operator give more care to samplepreparation so as to avoid any biasing of grain-size esti-mates, and he or she should use reflected light or a bin-ocular microscope to identify the sandy components ofthe rock.

CONCLUSION: DEPOSITION OF UNITS 8 TO 10

On the basis of our textural and compositional analy-ses of samples from Units 8 and 10 and shipboard de-scriptions of samples from Unit 9, we conclude that thedeposition of these units most likely occurred in shal-low, semirestricted to open-marine environments duringa marine transgression.

The basal Unit 10 was apparently deposited in veryshallow nearshore waters. The presence of large amountsof mud, pyrite, and organic debris suggests rapid depo-sition in fairly low-energy, semirestricted waters of a la-goon, bay, or protected shelf rather than an open-ma-

rine shelf subjected to wave and tidal reworking. Clasticsediments in Unit 10 were presumably supplied fromcoastal deltaic sources, as indicated by their textural andcompositional immaturity and the presence of abun-dant kaolinite (Weaver, 1958). The source of the carbon-ate mud, as always, is a controversial topic; opinions onthe subject vary from in situ production by algae (Stock-man et al., 1967) to abrasion of sand-sized clasts at ashelf margin and subsequent transport into a lagoon orbay (Bathurst, 1972; Pettijohn, 1975). Because of theamount of terrigenous mud in the section, it appearsthat conditions were not optimal for in situ productionof all the carbonate mud in this unit. Rather, much of itwas likely created by abrasion and then transported infrom shelf margin facies (represented by Unit 9) by waveand storm washover.

Within Unit 10, the lowermost subunit (lOd) wasdeposited in a distal deltaic environment proximal to acarbonate mud-rich zone. Distal deltaic sedimentationwould have led to the deposition of the noncalcareousmudstone and sandy mudstone beds. Occasional stormwashovers in turn would have deposited biogenic debristo produce the marly wackestone and calcareous sandymudstone beds found in this subunit. Subunit 10c, whichis composed largely of terrigenous and organic debrisand pyrite and rare beds of calcareous sediments, wasprobably deposited in a similar, although somewhat morerestricted or landward, environment than was SubunitlOd. In Subunit 10b, coarser carbonate lithologies areregularly interbedded with noncalcareous mudstones. De-

935


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Figure 14. SEM imagery of recrystallized and detrital clays. A. Detrital illite coating sand grain in Sample54-3, 128-131 cm. B. Same sample showing development of feathery illite recrystallized from the detri-tal illite. Clay identified by dispersive analysis. C. Recrystallized illite from Sample 89-1, 125-128 cm.D. Recrystallized kaolinite (arrow) and chlorite (in background) in Sample 79-1, 53-55 cm.

position of this subunit occurred farther seaward thaneither of the lower subunits, closer to the source of car-bonate debris (the shelf margin). Lastly, Subunit 10a,which contains more, particularly coarse-grained, car-bonate debris and less terrigenous mud than any lowersubunit, was probably deposited closest to the shelf mar-gin and farthest from the clastic-dominated shoreline.

On the basis of onboard descriptions, Unit 9 wasprobably deposited as milliporoid boundstones and skele-tal grainstones at a high-energy, well-oxygenated shelfmargin. These carbonate sediments presumably formeda bank that absorbed wave, tidal, and storm energy andprotected the more landward environments (representedby Unit 10) from these high-energy sources.

Unit 8 represents the final phase of marine transgres-sion. A combination of increased water depth, landwardshoreline migration, and the presence of a carbonate bankresulted in a sharp decrease in clastic sediment influx

and the deposition of carbonate wackestones and mud-stones seaward of the shelf margin. The upward grada-tion from highly fossiliferous wackestones to nonfos-siliferous, fine-grained micritic mudstones in this unitsuggests deposition in progressively deeper and less pro-ductive foreslope environments, where most of the sedi-ment was derived from downslope movement of shelfmargin debris.

ACKNOWLEDGMENTS

The authors wish to thank the Department of Geology, Texas A & MUniversity and the Mini-Grant Committee of Texas A & M Universityfor their financial support for this project. We also wish to thank R. J.Stanton and T. T. Tieh for their constructive comments.

REFERENCES

Bathurst, R. G. C , 1972. Carbonate Sediments and Their Diagenesis:New York (American Elsevier).

Carroll, D., 1978. Clay minerals: a guide to their X-ray identification.Spec. Pap. Geol. Soc. Am., 126.

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Pettijohn, F. J., 1975. Sedimentary Rocks: New York (Harper and Wilson, M. D., and Pittman, E. D., 1977. Authigenic clays in sand-Row), stones: Recognition and influence on reservoir properties and pa-

Stockman, K. W., Ginsburg, R. N., and Shinn, E. A., 1967. The pro- leoenvironmental analysis. J. Sed. Pet., 47:3-31.duction of lime mud by algae in south Florida. J. Sed. Pet., 37:633-648.

Weaver, C. E., 1958. Geologic interpretation of argillaceous sediments. Date of Initial Receipt: December 6, 1982Am. Assoc. Pet. Geol. Bull., 42:254-309. Date of Acceptance: July 18, 1983

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